The present invention relates to a process for the production of moldings of plastic reinforced with long fibers in which a continuous thread or roving is taken off from a coil, conveyed via a pipeline system to a cutting unit where it is cut into finite sections which form long fibers. These long fibers are fed via a conveying channel to a free-flowing stream which forms a solid plastic and combined with the free-flowing stream. This stream of plastic-forming material combined with the long fibers is introduced into a mold, the mold is closed and the finished molding is removed from the mold after curing. The present invention also relates to a device useful for carrying out the process of the present invention.
It is known to produce moldings of plastic reinforced with long fibers by incorporating long fibers in homogeneous distribution into a liquid plastic melt before curing or into a free-flowing reaction mixture before complete reaction of such mixture. xe2x80x9cLong fibersxe2x80x9d as used herein means those fibers obtained by cutting continuous threads or rovings (particularly glass fiber rovings) to a finite length of preferably more than 10 mm shortly before introduction into the plastic melt or into the free-flowing reaction mixture (DE 196 18 393 A1; WO 98/01274; WO 96/35562). The ends of the continuous threads or rovings wound up on coils are linked to one another by splicing so that no interruption takes place during take-off. The continuous thread or roving is fed from the bearing seat of the coil optionally, partly through free space or through a pipeline system. A conveying air feed is introduced at one or more points in this pipeline system. The continuous thread is conveyed pneumatically with the conveying air feed to the cutting unit. The cut threads are also generally conveyed pneumatically to the location at which they are introduced into the stream of liquid plastic.
It is possible to distribute the resulting mixture of liquid plastic and long fibers manually into the lower mold half in a primitive manner, but this usually does not satisfy the technical requirements for the molding to be produced. For introduction of a liquid plastic melt to which long fibers have been added or of a reaction mixture to which long fibers have been added into an open mold, a program-controlled introducing device, e.g. a robot, which distributes the melt or mixture uniformly in the lower mold half is therefore generally used. The mold is then closed and locked. The molded article cures in the mold and is then removed from the mold.
This known process has two problems. First, during distribution of the mixture in the mold cavity, either more or fewer long fibers are required due to the movement of the introducing device. The demand for continuous thread or roving also varies accordingly. Second, feeding or conveying not only of the long fibers but also of the continuous thread or roving is interrupted from mold filling to mold filling due to the discontinuous procedure. As a result, the continuous thread is often not braked as rapidly as desired and loops of thread or roving form. To avoid such loops, tension rolls may be used. When long fibers are introduced into liquid plastic melts with a relatively long solidification time, a take-off speed of the continuous thread of from about 0.5 to approx. 2 m/s is usual. As a result, the process usually just proceeds without problems which are too great. However, malfunctions due, for example, to tearing of the continuous thread cannot be ruled out.
It is an object of this invention is to provide a process and a device in which a trouble-free feed of the continuous thread or roving, particularly at higher take-off speeds, is ensured during production of moldings of plastics reinforced with long fibers.
These and other objects of the present invention which will be apparent to those skilled in the art are accomplished by conveying a continuous thread or roving to the cutting unit under tension pneumatically in a direction opposite to the direction in which the thread or roving is conveyed to the cutting device.